1. Field of the Invention
The present invention generally relates to welding. More specifically, the present invention is directed to a weld filler composition for use in welding together steel pieces with different alloy content to reduce the stress between the joints of the steel pieces.
2. Description of Related Art
In many high temperature piping and tubing applications, it is frequently necessary to join materials with different chemical compositions. As equipment operating temperatures and pressures increase, piping and tubing components require higher alloyed materials. Yet, it is more expensive to produce entire components from higher alloyed materials, so a combination of low alloyed and higher alloyed materials are often employed as a means to reduce cost. This cost-savings measure requires the ability to join materials with different alloy contents. For example, it may be desirable to weld ferritic steel, which is a low alloy steel, and austenitic steel, which is a high alloy content steel.
However, there are issues regarding components that are made by welding materials with different compositions. For example, components utilized in high temperature applications frequently experience thermal gradients between operation and shut down times. Since there is a difference in the coefficient of thermal expansion for each material, high stresses at the weld joint may be experienced as a result of such thermal gradients.
To address the high stresses experienced at the weld joint, various approaches for joining different materials have been attempted in the past. One approach is to use a filler metal that has a coefficient of thermal expansion that is in between the coefficient of thermal expansion of the two different materials. Another approach is to form a joint with a continuously changing chemical composition from one joined piece to the other, for example, by electroslag techniques. Another approach is to employ the use of powder metallurgy components to form these joints. Heat treatment has also been employed both prior to and following the formation of these joints to better condition the transition joint and interconnecting bonds to withstand severe operating conditions. Yet, these techniques have been met with mixed results, leaving room for improvement.
Another technique includes the use of stepped transition joints, in which a member is pre-formed from a plurality of sections that are welded together. The sections of the member are selected to generally provide a gradient of chemical composition and physical properties between the materials being joined together. For instance, the chromium content and the thermal coefficient of expansion of several segments are progressively graded between one end of the member and the other. While such stepped transition joints have generally been found to work well, multiple welded members are relatively expensive.
In the utility industry, the service life of weld joints between dissimilar materials has been less than desired. In addition to the problems associated with the differences between the coefficient of thermal expansion in each material, carbon migration causes short service life in these joints. Carbon migration occurs whenever the weld joint is subjected to elevated temperature conditions. The primary area of concern for this phenomenon is in the interface between ferritic steel and a filler metal. Carbon tends to increase in concentration in the weld or filler metal proximate to the base metal. Carbon generally transfers from the base metal to the filler metal producing a structure of weak carbides in the filler metal and a weak zone in the base metal having a lower carbon concentration.
Another problem that plagues the weld joint of dissimilar materials is micro-fissuring. Over time, micro-fissuring creates stress and fatigue in the weld joint, ultimately leading to its eventual failure. By reducing or eliminating micro-fissuring, the life of the weld joint can be prolonged. The longer life of a weld joint can translate into a substantial cost savings over the life of various equipment or plumbing parts.
Therefore, an improved filler composition or weld formulation for joining dissimilar materials is highly desired. Such a filler composition may address the problems associated with welded joints between dissimilar materials such as issues related to the difference in the coefficient of thermal expansion, carbon migration and micro-fissuring.